fix segmentation fault when meeting 'select * from t where 2 > 1' (#114)

src/observer/sql/executor/execute_stage.cpp

@@ -290,6 +290,8 @@ IndexScanOperator *try_to_create_index_scan_operator(FilterStmt *filter_stmt)
     if (left->type() == ExprType::FIELD && right->type() == ExprType::VALUE) {
     } else if (left->type() == ExprType::VALUE && right->type() == ExprType::FIELD) {
       std::swap(left, right);
+    } else {
+      continue;
     }
     FieldExpr &left_field_expr = *(FieldExpr *)left;
     const Field &field = left_field_expr.field();